1. Field of the Invention
The present invention relates to the construction and composition of optical fiber cables and laser systems utilizing such cables that provide for the ability to transmit high power laser energy over great distances, which distances and power transmission levels were heretofore believed to be unobtainable. The present invention further relates to the construction of such cables to withstand harsh environments. In particular, the present invention relates to a unique and novel combination of an optical fiber and a multi-layered structure for such cables.
As used herein, unless specified otherwise “high power laser energy” means a laser beam having at least about 5 kW (kilowatt) of power. As used herein, unless specified otherwise “great distances” means at least about 500 m (meter). As used herein the term “substantial loss of power” and “substantial power loss” means more than about 2.0 dB/km (decibel/kilometer) for a selected wavelength. As used herein the term “substantial power transmission” means at least about 50% transmittance.
2. Discussion of Related Art
Until the present invention, it was believed that a paradigm existed in that the transmission of high power laser energy over great distances without substantial loss of power was unobtainable. As a consequence, it was further believed that there was no reason to construct, or investigate the composition of, an optical fiber or an optical fiber cable for the transmission of high power laser energy over great distances.
Power loss over long distances occurs in an optical fiber from many sources including: absorption loss, and in particular absorption loss from hydroxyl ions (OH−); Rayleigh scattering; Brillouin scattering; Raman scattering; defects; inclusions; and bending loss. These problems have been documented in the literature.
For example, in the 2006, Crystal Fiber White Paper, titled “Towards 100 kW fiber laser system Scaling up power in fiber lasers for beam combining” it is provided, at page 4, that for stimulated Brillouin scattering (SBS) “the threshold scales as the square of signal mode field diameter (MFD) and inversely with the effective fiber length. Hence, larger core size and short fiber length are desired for power scaling.” (emphasis original) In Corning paper, NIST-SOFM 2004, titled “Stimulated Brillouin Scattering: An Overview of Measurements, System Impairments, and Applications” it is provided, at page 1, that “[o]f the three types of scattering events [Rayleigh, Raman and Brillouin] stimulated Brillouin scattering (SBS) is recognized as the dominant optical fiber nonlinearity.” (bracketed matter added) The Corning paper, at page 3, goes on to provide that “[t]he output power curve . . . also shows that the signal power becomes depleted beyond a certain input power. This deleterious result will effectively clamp the signal output power, but continue to transfer power to the Stokes (reflected) signal via the electrostrictive process which underlies the stimulated Brillouin phenomenon.” Thus, the Corning paper, at page 4, provides that “[s]timuated Brillouin scattering is known to grossly limit the design of several optical transmission systems, amplifiers, and lasers.”
This perceived paradigm, expressed in the art to be believed to exist between length of fiber and power transmittance is further illustrated in the May 31, 2007, Vol. 5, Supplement, pages S39-S41, CHINESE OPTICS LETTERS, Muto et al., titled “Laser cutting for thick concrete by multi-pass technique”, although Muto states that 4 kW of power were delivered down a 1 km fiber, when 5 kW of laser power was put into the fiber, Muto, however, fails to eliminate the stimulated Raman scattering SRS phenomena. As shown by Muto's paper this deleterious phenomenon will effectively clamp the output power as length or power is increased. The SRS phenomenon is seen by the spectrum that is shown in FIG. 3 of Muto, which figure is provided herein as FIG. 2 in this specification. In FIG. 2 the laser beam is shown as band 200 and the SRS is shown as band 201. Thus, prior to the present invention, it was believed that as input laser power, or the length of the fiber increased, the power output of a fiber would not increase because of the SBS, SRS and other nonlinear phenomenon. In particular, SBS would transfer the output power to back up the fiber toward the input. Further, SBS, SRS, as well as the other deleterious nonlinear effects, in addition to limiting the amount of power that can be transmitted out of the fiber, can result in fiber heating and ultimate failure. Thus, as recognized by Muto, at page S41 “[i]t is found that 10-kW power delivery is feasible through a 250-m-long fiber with the core diameter of 150 μm. The physical phenomenon which restricts the transmitted power is SRS.” Thus, Muto, as did others before him, failed to deliver high power laser energy over great distances.
The present invention breaks this length-power-paradigm, and advances the art of high power laser delivery beyond this paradigm, by providing an optical fiber cable laser system that overcomes these and other losses, brought about by nonlinear effects, and provides for the transmission of high power laser energy over great distances without substantial power loss.